Project 3 - SUMMARY Background: Invasive postpartum group A streptococcal (GAS) infections have re-emerged worldwide. Correct diagnosis is often delayed until infection has spread throughout the reproductive system resulting in increased maternal morbidity, prolonged hospitalization, and reproductive sterility. Observations that high levels of estrogen have profound effects on macrophage pro-inflammatory cytokine production suggest that the unique environment of the healing postpartum uterus may contribute to its vulnerability to GAS infection. Hypothesis: We hypothesize that GAS postpartum infections are mediated by estrogen-induced priming of decidual macrophages for anti-inflammatory (M2 healing) activity such that pro-inflammatory (M1 antimicrobial) host immune responses to GAS are suppressed. Specific Aim 1: To determine whether high estrogen levels at parturition drive decidual macrophages in the uterus and cervix to an M2 phenotype. Thus this study will utilize a murine postpartum model to determine the effect of pharmacologic blockade of estrogen receptors on macrophage phenotype switching. Specific Aim 2: To determine whether estrogen prolongs the time required for M2 macrophages to switch to a pro-inflammatory M1 phenotype. Using our methods for macrophage phenotype switching in vitro, the direct effects of estrogen on cytokine-driven macrophage phenotype switching from M2 to M1 will be determined. Specific Aim 3: To determine whether M2 macrophages in the postpartum uterus predispose to GAS postpartum infection. Since it is not presently possible to specifically delete M2 populations in vivo, IFNg-deficient mice will be used to elucidate the role of M2 macrophages in predisposing to GAS postpartum infections. Aims 1 and 3 will rely heavily on the services and expertise of the HPIC facility. Impact on Human Health: Understanding how estrogen modulates the host-pathogen interaction may reveal novel strategies for prevention or treatment of this devastating infection. Contribution to Multi-disciplinary Infectious Diseases Research Program: The PI, Dr. Li, possesses an in-depth knowledge of innate immune cell function and the skills for isolating and characterizing cells of interest from complex tissues. She also brings an in-depth molecular understanding of intracellular signaling pathways engaged in response to infection. Under her direction, the proposed studies will establish a novel experimental model to study host/pathogen interactions at the cellular and molecular levels in the dynamic setting of the postpartum uterus. These studies will broaden the COBRE team's focus to include infections of particular relevance to women's health and will increase the core's expertise in molecular and cellular immunologic defense mechanisms.